DC offset cancellation

ABSTRACT

A transmitter has a transmission path with an IQ modulator and a feedback path with an IQ demodulator. DC offset is determined by estimating a DC offset of the IQ demodulator, adaptively estimating a DC offset of the IQ modulator at least partially from the demodulator DC offset, subtracting the estimated demodulator DC offset from the feedback path and subtracting the estimated modulator DC offset from the transmission path.

FIELD OF THE INVENTION

The present invention relates to offset cancellation in general and toDC offset cancellation in mobile communication systems in particular.

BACKGROUND OF THE INVENTION

Many transmitters transmit digital information values that are generatedin base band. The base band digital values are modulated onto a carrierhigh frequency signal and the combined signal is amplified before itstransmission. The base band values may be complex values having real andimaginary components which are traditionally referred to as I and Qcomponents, respectively. In the modulation of the base band signal andamplification of the modulation signal, inaccuracies are introduced.These inaccuracies may cause the transmitter to interfere with signalson carrier frequencies allocated to other transmitters and thereforeshould be at least partially canceled by the transmitter.

One source of inaccuracy is the IQ modulator and demodulator, which bothsuffer from a distortion mechanism called “local oscillator carrierfeedthrough” and “DC offset”. For example, the output of the demodulatorcan be modeled as:

S(t)=I(t)cos(ωt)−Q(t)sin(ωt)+LO(t)  Equation 1

where:

LO(t)=A cos(ωt+φ).  Equation 2

The DC offset mechanism is generally due to LO(t), which is caused dueto leakage of the signal of a local oscillator (used for carriermodulation) into the demodulator output. The same problem occurs at themodulator side.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with theappended drawings in which:

FIG. 1 is a block diagram illustration of transmission and feedbackpaths of a mobile communication unit or a base station, in accordancewith an embodiment of the present invention; and

FIG. 2 is a block diagram illustration of an alternative embodiment ofthe present invention; and

FIG. 3 is a flowchart illustration of a method in accordance with anembodiment of the present invention.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the presentinvention.

Some portions of the detailed description which follow are presented interms of algorithms and symbolic representations of operations on databits or binary digital signals within a computer memory. Thesealgorithmic descriptions and representations may be the techniques usedby those skilled in the data processing arts to convey the substance oftheir work to others skilled in the art.

An algorithm is here, and generally, considered to be a self-consistentsequence of acts or operations leading to a desired result. Theseinclude physical manipulations of physical quantities. Usually, thoughnot necessarily, these quantities take the form of electrical ormagnetic signals capable of being stored, transferred, combined,compared, and otherwise manipulated. It has proven convenient at times,principally for reasons of common usage, to refer to these signals asbits, values, elements, symbols, characters, terms, numbers or the like.It should be understood, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities.

Unless specifically stated otherwise, as apparent from the followingdiscussions, it is appreciated that throughout the specificationdiscussions utilizing terms such as “processing,” “computing,”“calculating,” “determining,” or the like, refer to the action and/orprocesses of a computer or computing system, or similar electroniccomputing device, that manipulate and/or transform data represented asphysical, such as electronic, quantities within the computing system'sregisters and/or memories into other data similarly represented asphysical quantities within the computing system's memories, registers orother such information storage, transmission or display devices.

Embodiments of the present invention may include apparatuses forperforming the operations herein. This apparatus may be speciallyconstructed for the desired purposes, or it may comprise a generalpurpose computer selectively activated or reconfigured by a computerprogram stored in the computer. Such a computer program may be stored ina computer readable storage medium, such as, but is not limited to, anytype of disk including floppy disks, optical disks, CD-ROMs,magnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), electrically programmable read-only memories (EPROMs),electrically erasable and programmable read only memories (EEPROMs),magnetic or optical cards, or any other type of media suitable forstoring electronic instructions, and capable of being coupled to acomputer system bus.

Reference is now made to FIG. 1, which generally illustrates elementsused in transmission for both mobile communication units and the basestations with which they communicate. While FIG. 1 presents certainelements, it will be appreciated that other mobile units and basestations may or may not include all of the elements shown in FIG. 1.FIG. 1 shows a transmission path 10, a feedback path 12 and a DC offsetestimator 16. The transmission path 10 generally comprises some or allof the following elements: a baseband modulator 20, a digital to analog(D/A) converter 24, an IQ modulator 28 and a power amplifier 30.Baseband modulator 20 converts an incoming bit stream into a basebandsignal having I and Q components. D/A converter 24 converts the shapeddigital signal into an analog signal. IQ modulator 28 modulates thecomplex baseband signal into a radio frequency (RF) signal and poweramplifier 30 transmits he RF signal.

Feedback path 12 comprises some or all of the following elements: anattenuator 32, an IQ demodulator 34 and an analog to digital (A/D)converter 40. Attenuator 32 receives the transmitted radio frequency andIQ demodulator 34 converts the radio frequency signal into a basebandone. Analog to digital converter 40 converts the signal into a digitalone.

DC offset estimator 16 generally determines the DC offset due to IQmodulator 28 and IQ demodulator 34 from data from feedback path 12. DCoffset estimator 16 typically comprises a demodulator DC offsetestimator 50 and an adaptive modulator DC offset estimator 52. Through alogical switch 54 in feedback path 12, demodulator estimator 50 mayreceive the output V_(f) of analog to digital converter 40 at predefinedtimes. Demodulator estimator 50 may estimate the DC offset due to IQdemodulator 34 and the result, a signal labeled DC_DEMOD_EST, may beprovided to a summer 56 in feedback path 12. When logical switch 54connects analog to digital converter 40 to summer 56, summer 56 maysubtract (block 300 of FIG. 3) the estimated DC offset, DC_DEMOD_EST,from the output of analog to digital converter 40, thereby providing adifference signal from which most, if not all, of the DC offset due toIQ demodulator 34 has been removed.

Typically, logical switch 54 provides its signal to demodulatorestimator 50 during “non-transmission slots” (i.e. periods when nosignal is being transmitted from or to the unit), At the same time, a“zero” signal, or one with a known sequence, may be injected to theinput of IQ demodulator 34. This may be implemented by shutting downpower amplifier 30 which results in an input to IQ demodulator 34 ofgenerally zero. Thus, if there is any signal measured after analog todigital converter 40, it is due to the DC offset of IQ demodulator 34.

The output of IQ demodulator 34 may be measured for a set number ofconsecutive symbols NO_SYMBOLS. The two DC offset values DC_I_DEMOD_ESTand DC_Q_DEMOD_EST of IQ demodulator 34 may be estimated as:$\begin{matrix}{{{DC\_ I}{\_ DEMOD}{\_ EST}} = {\frac{1}{NO\_ SYMBOLS}{\sum\limits_{i = 1}^{NO\_ SYMBOLS}\overset{\sim}{I}}}} & {{Equation}\quad 3}\end{matrix}$

$\begin{matrix}{{{{DC\_ Q}{\_ DEMOD}{\_ EST}} = {\frac{1}{NO\_ SYMBOLS}{\sum\limits_{i = 1}^{NO\_ SYMBOLS}\overset{\sim}{Q}}}},} & {{Equation}\quad 4}\end{matrix}$

where Ĩ, {tilde over (Q)} are the real and imaginary parts,respectively, of the signal V_(f) produced by analog to digitalconverter 40 and NO_SYMBOLS is the number of symbols used for averaging.In one embodiment, NO_SYMBOLS=5.

These two values may be provided to summer 56 to generally cancel the DCoffset of IQ demodulator 34.

Once the demodulator DC offset has been estimated, the modulator DCoffset may be calculated by operating both transmission path 10 andfeedback path 12. Transmission path 10 also includes a summer 60, whichmay subtract the modulator DC offset, DC_MOD_EST, from the signalproduced by baseband modulator 20. To determine the modulator DC offseta zero input signal is provided (typically during a non-transmissionslot) to baseband modulator 20. In addition, adaptive modulatorestimator 52 may assign initial values, typically obtained from afactory calibration, to summer 60. The factory calibrated values may befar away from the true (and unknown) values due to thermal and frequencychanges.

The resultant signal is transmitted by power amplifier 30 and may bereceived by feedback path 12, operating with logical switch 54connecting analog to digital converter 40 to summer 56. The result isthat summer 56 produces two signals e_(I) and e_(Q) which are thefeedback path output with the DC offset from IQ demodulator generallyremoved. During non-transmission slots, the two signals may be givenmathematically as:

e_(I)=Ĩ-DC_I_DEMOD_EST,  Equation 5

e_(Q)={tilde over (Q)}-DC_Q_DEMOD_EST,  Equation 6

where Ĩ, {tilde over (Q)} are defined hereinabove.

When determining the DC offset of IQ modulator 28, a logical switch 58may provide the two signals e_(I) and e_(Q) to adaptive modulatorestimator 52, which then operates to minimize the signals e_(I) ² ande_(Q) ².

At the end of each symbol period, adaptive modulator estimator 52 maycompute (block 306 of FIG. 3):

DC_I_MOD_C_(new)=DC_I_MOD_C_(old)-μ_(I) Re{e^(rot) _(I)},  Equation 7

for the I channel and

DC_Q_MOD_C_(new)=DC_Q_MOD_C_(old)-μ_(Q) Im{e^(rot) _(Q)},  Equation 8

for the Q channel, where μ_(I),μ_(Q) are the step sizes which controlthe rate of adaptation, 0 ≦μ_(I)≦1, 0≦μ_(Q)≦1 and e^(rot) _(I) ande^(rot) _(Q) are rotated errors given hereinbelow by Equation 9.DC_I_MOD_C_(old) and DC_Q_MOD_C_(old) are the previous values of offsetestimation and initially may be the factory calibrated values.

Alternatively to using the steepest descent iteration method(implemented in Equation 7 and Equation 8), other iteration methods asare known from optimization theory, such as the conjugate gradientequation and the quasi-Newton equation, may be used to minimize thesignals e_(I) ² and e_(Q) ². In some embodiments of the invention, theselected iteration method is chosen as a tradeoff between the processingpower of the transmitter and the required convergence speed of theoutput of adaptive estimator 52. If fast convergence is required and thetransmitter has a relatively high processing power level, a fastconvergence method that requires dense computation (e.g., thequasi-Newton method) may be used. If, however, low processing powerutilization is more important than fast convergence, simpler methods,such as the steepest descent method, may be used.

There is often a phase rotation {circumflex over (φ)}_(path) along thepath from the output of digital to analog converter 24 in transmissionpath 10 to the input to analog to digital converter 40 in feedback path12. This phase rotation can be estimated (block 302 of FIG. 3) asdescribed hereinbelow with respect to Equation 10 and then used torotate (block 304 of FIG. 3) the errors e_(I) and e_(Q) according to:

e^(rot) _(I)=e_(I)e^(−j{circumflex over (φ)}) ^(_(path))

e^(rot) _(Q)=e_(Q)e^(−j{circumflex over (φ)}) ^(_(path))   Equation 9

The phase {circumflex over (φ)}_(path) is measured in the firsttransmission slot when data is transmitted. The angle θ_(bip) of thebaseband input signal is measured, per symbol, as is the angle θ_(out)of each symbol of the signal V_(f) after analog to digital converter 40.The angle is defined as the angle in the complex plane of valueĨ+j{tilde over (Q)}.

The phase {circumflex over (φ)}_(path) is the average value of thedifference in the angles. Thus:

{circumflex over (φ)}_(path=avg(θ) _(out)−θ_(bip))  Equation 10

During regular operation, logical switches 54 and 58 are typicallyclosed and the previously estimated DC offset values may be utilized insummers 56 and 60. Thus, feedback path 12 produces signals with minimal,if any, demodulator DC offset and the transmitted signal may have themodulator DC offset removed.

If desired, and particularly during long transmission periods, the DCoffset estimators 50 and 52 can be operated again. To do so, the data ofthe next non-transmission slot is used. The DC offset estimators 50 and52 are then operated on this data.

Reference is now made to FIG. 2, which illustrates a further embodimentof the invention implemented in a transmitter having a predistorter.Elements, which are similar to those of FIG. 1, have similar referencenumerals.

FIG. 2 shows transmission path 10, feedback path 12, a predistorter 14and DC offset estimator 16. The transmission path 10 generally comprisessome or all of the following elements: baseband modulator 20, a finiteimpulse response (FIR) filter 22, digital to analog (D/A) converter 24,an analog filter 26, IQ modulator 28 and power amplifier 30. FIR filter22 shapes the baseband signal as desired. Analog filter 26 filters theanalog signal as necessary.

Feedback path 12 comprises some or all of the following elements:attenuator 32, IQ demodulator 34, a filter 36 and analog to digital(A/D) converter 40. Filter 36 limits any noise to the bandwidth of thedemodulated signal.

Predistorter 14 comprises a predistorter (PD) lookup table (LUT) 42 anda PD LUT trainer 44. Pre-distorter 14 compensates for the non-linearityof power amplifier 30 and changes the signals entering power amplifier30 such that the transmitted signals have substantially linearamplification (rather than the non-linear amplification, which occurswithout the predistortion). Since the distortion changes due totemperature, aging and other characteristics of power amplifier 30, thepredistortion values are updated by PD LUT trainer 44, based on feedbackreceived from the output of power amplifier 30.

During regular transmission, PD LUT 42 predistorts the signal frombaseband modulator 20 in order to compensate for the distortion producedby power amplifier 30. To do so, the output of PD LUT 42 is multipliedwith the output of baseband modulator by multipliers 46 in transmissionpath 10. PD LUT trainer 44 regularly updates the values of PD LUT 42based on data received along feedback path 12.

During regular operation, logical switches 54 and 58 are typicallyclosed and any previously estimated DC offset values may be utilized insummers 56 and 60. Thus, PD LUT trainer 44 may receive signals withminimal, if any, demodulator DC offset and the transmitted signal may beboth predistorted and may have the modulator DC offset removed.

DC offset estimator 16 operates as described hereinabove. Specifically,it operates during non-transmission slots and PD LUT 42 is disabled byindicating to multipliers 46 to pass the output of baseband modulator 20rather than multiplying it by the output of PD LUT 42.

The methods and apparatus disclosed herein have been described withoutreference to specific hardware or software. Rather, the methods andapparatus have been described in a manner sufficient to enable personsof ordinary skill in the art to readily adapt commercially availablehardware and software as may be needed to reduce any of the embodimentsof the present invention to practice without undue experimentation andusing conventional techniques.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed herein above. Rather the scope of the invention is defined bythe claims that follow:

What is claimed is:
 1. A method for reducing a DC offset in atransmitter, the transmitter having a transmission path with an IQmodulator and a feedback path with an IQ demodulator, the methodcomprising: providing a predetermined signal to the input of said IQmodulator and transmitting said signal; estimating a DC offset of saidIQ demodulator from a received version of said transmitted signal;adaptively estimating a DC offset of said IQ modulator at leastpartially from said demodulator DC offset; subtracting said estimateddemodulator DC offset from said feedback path; and subtracting saidestimated modulator DC offset from said transmission path, wherein saidestimating said demodulator DC offset comprises averaging a receivedsignal along said feedback path, and wherein adaptively estimating saidmodulator DC offset comprises generating a difference signal betweensaid received signal and said estimated demodulator DC offset andsubtracting a modified rotated version of said difference signal from aprevious value of said modulator DC offset.
 2. A method according toclaim 1 and wherein estimating said demodulator DC offset and adaptivelyestimating said modulator DC offset occur during non-transmission slots.3. A method according to claim 2 and wherein said non-transmission slotsare non-transmission slots of regular transmission.
 4. A methodaccording to claim 1 wherein adaptively estimating said modulator DCoffset comprises estimating a phase rotation of said transmission andfeedback paths and modifying said difference signal with said phaserotation.
 5. A transmitter comprising: a transmission path having an IQmodulator and a first summer; a feedback path having an IQ demodulatorand a second summer; a demodulator DC offset estimator to average areceived signal along said feedback path and to estimate a DC offset ofsaid IQ demodulator; an adaptive modulator DC offset estimator toestimate a DC offset of said IQ modulator at least partially from saiddemodulator DC offset, wherein said second summer is to subtract saidestimated demodulator DC offset from said feedback path, wherein saidfirst summer is to subtract said estimated modulator DC offset from saidtransmission path, and wherein said adaptive modulator DC offsetestimator is to generate a difference signal between said receivedsignal and said demodulator DC offset and to subtract a modified rotatedversion of said difference signal from a previous value of saidmodulator DC offset.
 6. A transmitter according to claim 5 and whereinsaid demodulator DC offset estimator and said adaptive modulator DCoffset estimator are to operate during non-transmission slots.
 7. Atransmitter according to claim 6 and wherein said non-transmission slotsare non-transmission slots of regular transmission.
 8. A transmitteraccording to claim 5 wherein said adaptive modulator DC offset estimatoris to modify said difference signal with a phase rotation of saidtransmission and feedback paths.
 9. A transmitter according to claim 5which forms part of a base station.
 10. A transmitter according to claim5 which forms part of a mobile communication unit.
 11. A DC offsetreducer comprising: a transmission path having an IQ modulator and afirst summer; a feedback path having an IQ demodulator and a secondsummer; a demodulator DC offset estimator to average a received signalalong said feedback path and to estimate a DC offset of said IQdemodulator; an adaptive modulator DC offset estimator to estimate a DCoffset of said IQ modulator at least partially from said demodulator DCoffset, wherein said second summer is to subtract said estimateddemodulator DC offset from said feedback path, wherein said first summeris to subtract said estimated modulator DC offset from said transmissionpath, and wherein said adaptive modulator DC offset estimator is togenerate a difference signal between said received signal and saiddemodulator DC offset and to subtract a modified rotated version of saiddifference signal from a previous value of said modulator DC offset. 12.A DC offset reducer according to claim 11 and wherein said demodulatorDC offset estimator and said adaptive modulator DC offset estimator areto operate during non-transmission slots.
 13. A DC offset reduceraccording to claim 12 and wherein said non-transmission slots arenon-transmission slots of regular transmission.
 14. A DC offset reduceraccording to claim 11 wherein said adaptive modulator DC offsetestimator is to modify said difference signal with a phase rotation ofsaid transmission and feedback paths.
 15. An integrated circuit (IC)having a transmitter, the transmitter comprising: a transmission pathhaving an IQ modulator and a first summer; a feedback path having an IQdemodulator and a second summer; a demodulator DC offset estimator toaverage a received signal along said feedback path and to estimate a DCoffset of said IQ demodulator; an adaptive modulator DC offset estimatorto estimate a DC offset of said IQ modulator at least partially fromsaid demodulator DC offset, wherein said second summer is to subtractsaid estimated demodulator DC offset from said feedback path, whereinsaid first summer is to subtract said estimated modulator DC offset fromsaid transmission path, and wherein said adaptive modulator DC offsetestimator is to generate a difference signal between said receivedsignal and said demodulator DC offset and to subtract a modified rotatedversion of said difference signal from a previous value of saidmodulator DC offset.
 16. An IC according to claim 15 and wherein saiddemodulator DC offset estimator and said adaptive modulator DC offsetestimator are to operate during non-transmission slots.
 17. An ICaccording to claim 16 and wherein said non-transmission slots arenon-transmission slots of regular transmission.
 18. An IC according toclaim 15 wherein said adaptive modulator DC offset estimator is tomodify said difference signal with a phase rotation of said transmissionand feedback paths.